1. Field of the Invention
The present invention relates to electronic systems, and more specifically to a system for controlling the rates of multiple transmissions having different rate constraints on the same communication channel.
2. Description of Related Art
“Traffic shaping”, or controlling the rates of multiple transmissions having different rate constraints on the same communication channel is used in devices such as a multiprotocol switch that acts as an interface between an ATM network and Ethernet networks. FIG. 1 shows a switch transmitting over an ATM network. The data of several concurrent transmissions, or open connections, reach a control circuit 10 that stores the data in a memory 12 before sending it in a controlled way over the ATM network via an adequate interface 14.
When a connection is created on an ATM network, there is an initial handshake between the transmitter and the network to determine, among other things, the transmission flow rate that will be used for the transmission. Indeed, the available transmission rate over an ATM network (for example, 155 Mbits/s) must be shareable in real time between all the possible connections. Thus, at the end of the handshake, a rate is assigned to the new connection, which rate must be respected or the network will loose its data.
Further, the handshake also concerns a type of connection which determines a “profile” or rate. The typical connection types are CBR (“constant bit rate”), VBR (“variable bit rate”) in real time or not, ABR (“available bit rate”), and UBR (“unspecified bit rate”). For CBR type connections, the network guarantees a fixed rate, which must not be exceeded. For real time VBR connections, the network determines a peak cell rate (PCR) and guarantees a sustainable cell rate (SCR) and a maximum burst size (MBS). The source can transmit at the peak cell rate PCR during a burst of the MBS at negotiated times and transmit at the SCR otherwise.
Further, for real time VBR connections, the network guarantees a cell delay variation objective (CDV). Connections of VBR type that are not in real time have the same features as real time VBR connections, but guarantee no cell delay variation objective CDV. For connections of the ABR type, the network determines the peak cell rate (PCR) and guarantees a minimal cell rate (MCR), with the transmission rate being likely to vary between the two limits. Finally, for UBR type connections, the network guarantees no rate. The network uses the “as good as possible” method.
A circuit controlling transmission over an ATM network must thus control the rates of the data transmitted over the ATM network to respect the negotiated bit rate profiles. For this purpose, conventional traffic shaping circuits associate, with the connections open in transmission, counters that count the number of data transmitted in order to deduce therefrom the bit rate and adjust the data transmission so that the connection is conformable to the negotiated parameters. The managing of these counters and the corresponding control result in complex control circuits. An example of such a control circuit is described in U.S. Pat. No. 5,724,513.